The invention relates to an earth protection device comprising an input to receive a first signal representative of an earth fault current, first signal processing means to process a signal representative of an earth fault current, and means for processing protection functions connected to the first signal processing means.
Known earth protection devices installed in trip devices or circuit breakers comprise means for detecting zero-phase sequence or earth fault currents. These currents are generally detected by making the vector sum of all the line currents, i.e. the phase currents and the neutral current.
This vector sum can be obtained by measuring currents separately in each line conductor with independent sensors, and a device then determines an earth fault current signal by making the sum of the measured signals. It is also possible to measure an earth fault current by using a sensor which surrounds all the conductors. A signal supplied by such a sensor is directly representative of an earth fault current.
The signals representative of earth fault current are used in protection functions to command opening of the circuit breaker contacts or to indicate the presence of earth fault current.
In known earth protection devices, the signals representative of fault currents are processed to extract an rms value of said signals. However, when the fault currents are caused by electric arcs, the duration of said currents is very short. The duration of arc currents is often much lower than the period corresponding to the frequency of an electrical power distribution system. These fault currents of short duration do not have a very high rms value but are of great nuisance in electrical installations. Moreover, these arc currents can present a danger for people using equipment connected to the power supply system.
Rms value detection does not enable these arc currents to be detected correctly.
Devices exist using peak value detections. These devices can detect arc currents correctly but they are not sufficiently precise to detect currents presenting harmonic components.
The object of the invention is to achieve an earth protection device enabling improved detection of earth fault currents comprising electric arc currents, and also a trip device and a circuit breaker comprising such a device.
A device according to the invention comprises second signal processing means to process a signal representative of an earth fault current connected between said input and the first signal processing means, said second signal processing means supplying to the first signal processing means a second signal representative of an earth fault current having on decrease a gradient lower than a preset gradient limiting value.
In a preferred embodiment, the second signal processing means comprise:
sampling means to sample the first signal representative of an earth fault current,
storing means to store a last value of the second signal representative of an earth fault current.
decrease calculating means to calculate a value with limited decrease connected to the storing means and to means for supplying a gradient limiting value, the value with limited decrease being representative of the last value of the second signal representative of an earth fault current less the gradient limiting value, and
determining means for determining a new value of the second signal representative of an earth fault current connected to the sampling means and to the decrease calculating means.
Advantageously the determining means comprise means for detecting a maximum to supply a new value of the second signal representative of an earth fault current, said second signal being determined according to a maximum value of the first sampled signal or of the limited decrease value.
Preferably the determining means comprise:
means for calculating a difference connected to the storing means and to the sampling means to determine the difference between a value of the first sampled signal and a last value of the second signal,
comparing means to compare said difference with the gradient limiting value and
selection means having inputs connected to the sampling means and to the decrease calculating means and being controlled by the comparing means to supply on output a signal having a limited decrease value if the difference between a value of the first sampled signal and a last value of the second signal is greater than the gradient limiting value.
In a particular embodiment, the gradient limiting value is constant.
According to a first alternative embodiment, the gradient limiting value is variable according to a last peak value of the first signal representative of an earth fault current.
According to a second alternative embodiment, the gradient limiting value is variable according to a time between a last peak value and a subsequent instantaneous value of the first signal representative of an earth fault current.
According to a third alternative embodiment, the gradient limiting value is variable according to a last peak value and to a time between a last peak value and a subsequent instantaneous value of the first signal representative of an earth fault current.
The device preferably comprises processing means implementing a process comprising:
a read step of an input signal representative of an earth fault current.
a step for determining a limited decrease value,
a selection step to supply an output signal representative of an earth fault current, said output signal being with limited decrease if the difference between the input signal and a previous output signal is greater than a limiting value, and
a storing step of the output signal.
Advantageously the process comprises a step for detection of a maximum value between the input signal and the limited decrease value.
An electronic trip device according to the invention comprises an earth protection device as defined above.
A circuit breaker according to the invention comprises an earth protection device as defined above.